A process in which a molten resin material is injected into a mold worked into a desired shape in advance and cooled and solidified in the mold, and the mold is then opened to take a molded product out of the mold, has been widely known as an injection molding process and utilized. A product shape portion for a molded product having a product function and an ingate (nozzle portion) of a molding machine for injecting a molten resin into a mold are generally connected through such portions as to be called a sprue, a runner and a gate. FIG. 23 illustrates a side-gate mold generally used. The mold includes an injection port 48 of a nozzle part, a gate 51, a runner 52, a sprue 53, a product shape portion 49 and an ejector 50. With the generally used side gate type mold illustrated in FIG. 23, the product shape portion, gate, runner and sprue are taken out of the mold by the ejector as one piece. Since these sprue, runner and gate portions have no product function and are unnecessary portions, these portions are discarded or reused by remelting or the like after being cut off from the product shape portion after molding. However, it takes a labor cost since a worker cuts off the gate portion using a cutting tool such as nippers after the sprue and runner portions are taken out of the mold. In addition, an automatic machine for cutting has involved a cost problem such as an increase in part cost due to the cost of the automatic machine. In order to solve these problems, a pin-gate method, in which a gate portion is formed in the shape of a small pin, and the gate is automatically cut off upon mold opening in removal of a molded product from a mold, is used. According to the pin-gate method, a gate can be cut without using a worker, and an exclusive machine is also unnecessary, so that such method can be widely used for small parts. In the pin-gate method, however, the mold is in the form called a three-plate mold, so that the pin-gate method has involved such problems as complication of the mold structure, increase in molding cycle and material cost due to increase in runner volume, and increase in mold thickness. As a method for solving the problem of the pin gate method, a hot-runner method, which requires neither a sprue nor a runner, has been proposed and put into practical use. According to the hot-runner method, a product is molded directly or by using a very small runner, so that a discarded material is very small. Further, a direct gate type in which a gate of a hot runner is directly provided on a product has the merit not only that no scrap material is produced but also that cutting of the gate is unnecessary. However, it is often difficult from the viewpoints of product function and design to directly provide the gate on the product shape portion, and moreover there arise such problems not only that increase in mold cost is brought because the hot runner itself is expensive but also that maintenance is hard because the mold structure is complicated.
As a method for solving the above-described problems, a method called an in-mold gate cutting method, in which a function of cutting a gate is provided in a mold, has been proposed in addition to a method called a side-gate method, in which the mold structure is very simple. JP Patent Registration No. 3041975 discloses that a gate cutting pin having a slope surface in a direction of a runner is provided, and the pin is moved in a direction perpendicular to the runner, thereby causing a resin in a gate portion to flow backward in the direction of the runner to cut off a product shape portion from the gate portion. JP Patent Registration No. 3178659 and JP Patent Registration No. 3207784 disclose an in-mold gate cutting method for a disc gate, which is employed for CDs, optical discs and DVD discs. According to these methods, a sprue cutting punch having a movable cutter and a cut hole, into which the end portion of the sprue cutting punch can be advanced upon operation thereof, are provided on the core side and the cavity side of the disc gate, respectively, whereby the disc gate is cut in the mold. JP Patent Application Laid-Open No. H09-174621 discloses a method in which a gate cutting pin having a slope surface provided on the side of a product is operated to push a resin in a gate portion toward the product, and a gate is cut off at the same time. U.S. Pat. No. 6,676,401 discloses a gate cutting pin operating unit provided in a mold and a gate-cutting pin with a slope surface provided on the side of a runner, in which the position of a cutting pin after cutting a gate comes into contact with both cavity and runner.
However, a high pressure of 20 Mpa to 80 Mpa, which is called a holing pressure after filling a resin, is generally applied to the runner for preventing defects such as sink marks, which occur at the product shape portion. In these conventional molds and methods, it is very difficult, for the method in which the slope surface is provided on the gate cutting pin and thus the gate cutting pin causes the resin in the gate portion to flow backward to the side of the runner, to return the resin to the side of the runner by the slope surface of the gate cutting pin before or just after completion of the pressure holding. There has also been the need of enlarging the size of the runner for returning the resin to the runner side and of slowing the cooling and solidification of the resin. However, the enlargement of the runner size has involved problems that a molding cycle is elongated because the runner cooling time is long and that a scrap material is increased by an amount corresponding to the enlargement. In order to prevent the resin from flowing backward during cooling after a sufficient pressure is applied to the product shape portion, it is an ordinary way that the mold is generally designed in such a manner that the thickness of the gate is made thinner than the thickness of a product to cool and solidify the gate portion faster than the product. However, it is necessary to conduct the cutting of the gate by the gate cutting pin before the gate is solidified, so that there arises such a problem of trade-off between the pressure holding and the timing of the gate cutting that a sufficient pressure cannot be applied to the product shape portion. In addition, when the thickness of the gate is made greater than the product shape portion, a problem that flash is produced on a gate-cutting surface on the side of the product has arisen in addition to the problems that the molding cycle is elongated and the scrap material is increased, like the case where the runner is enlarged. In general, for all the conventional methods, since a working surface of the gate cutting is a part of a product surface and becomes a sliding surface of the cutting pin, scuff marks and discontinuous irregularities occur in a part of the product surface. Thus they involve such defects that it is difficult to retain the product surface quality at the gate equivalent to that of the product surface around the gate.